The internal structure of jets has been an active research topic in QCD in recent years. In this talk, we propose to use one particular jet substructure - the so-called jet fragmentation function to study spin-dependent distribution and dynamics. In particular, we provide the general theoretical framework for studying the distribution of hadrons inside a jet by taking full advantage of the transverse-momentum-dependent distributions and polarization effects. The key development referred to as “polarized jet fragmentation functions’’, opens up new opportunities to study transverse momentum dependent (TMD) fragmentation functions via jets. Besides providing the theoretical understanding for the well-known Collins asymmetry for hadron in a jet, we also give additional examples involving polarization of Lambda baryons and pions inside the jet at the future Electron-Ion Collider.